CHICAGO, (Reuters) – A study using new imaging  technology found “silent” heart attacks may be far more common,  and more deadly, than suspected, U.S. researchers said yesterday.

Some studies estimate that these often painless heart attacks,  also known as unrecognized myocardial infarctions, affect 200,000  people in the United States each year.

But Dr. Han Kim of Duke University in North Carolina suspects  the numbers may be far higher.

“No one has fully understood how often these heart attacks  occur and what they mean, in terms of prognosis,” Kim, whose study  will appear next week in the Public Library of Science journal  PLoS Medicine, said in a statement.

Doctors usually can tell whether a patient has had a recent  heart attack by looking for signature changes on a test of the  heart’s electrical activity called an electrocardiogram and by  checking for certain enzymes in the blood.

For a heart attack that might have occurred in the past,  doctors look for changes on an electrocardiogram called a Q-wave,  a marker for damaged tissue.

But not all silent heart attacks result in Q-waves.

“Those are the ones we haven’t been able to count because  we’ve never had a good way to document them,” Kim said.

To spot these, Kim and colleagues used a new type of magnetic  resonance imaging technology called delayed enhancement  cardiovascular magnetic resonance, which is especially adept at  finding damaged heart tissue.

They studied 185 patients with coronary artery disease but no  record of heart attacks who were scheduled to have a test to look  for possible blockages in their heart arteries.

They found that 35 percent of the patients had evidence of a  prior heart attack. And they found that these so-called non-Q-wave  heart attacks were three times more common than silent heart  attacks with Q-waves.

They also found that after two years of follow up, people who  had suffered a silent, non-Q-wave heart attack had an 11-fold  higher risk of death from any cause and a 17-fold higher risk of  death due to heart problems, when compared to patients who did not  have any heart damage.

Kim said currently people who have had silent heart attacks  are treated like other patients with heart disease.

But given the findings, he said new studies should look at the  best way to care for these patients.

Heart disease is the No. 1 cause of death in the United  States, followed by cancer and stroke.